1. Field of the Invention
In at least one embodiment, the present invention relates metal-organic frameworks with high levels of surface area and porosity.
2. Background Art
Porous materials have become important in a number of chemical and physical processes which include, for example, gas/liquid separation, catalysis, luminescence-based sensors, gas storage, and the like. Typically, each specific application requires a tailoring of the pore size and the atomic and molecular adsorption properties to achieve a desired result. One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas. Until recently the highest surface area for a disordered structure was that of carbon (2,030 m2/g), and for an ordered structures was that of zeolite Y (904 m2/g). More recently crystals of metal-organic frameworks (“MOFs”) with similar or somewhat higher surface areas have been reported. Despite this progress and the critical importance of high surface area to many applications involving catalysis, separation and gas storage, no strategy has yet been outlined to answer the question of what the upper limit in surface area for a material is, and how it might be achieved.
Methods for tailoring pore size and adsorption involve altering chemical composition, functionality, and molecular dimensions without changing the underlying topology. (See A. Stein, S. W. Keller and T. E. Mallouk, Science 259, 1558 (1993); and P. J. Fagan and M. D. Ward, Sci. Am. 267, 48 (1992).) Although desirable, few systematic approaches exist because of the lack of control over molecular assembly and in particular, the inability to control the orientation of atomic groups in crystals. These difficulties should be contrasted with the synthesis of organic molecules which can be formed by well characterized and controllable steps. Typically, the insolubility of extended solids requires that assembly of these materials be accomplished in a single step. (See O. M. Yaghi, M. O'Keeffe, and M. Kanatzidis, J. Solid State Chem. 152, 1 (2000).)
Stable, porous metal-organic frameworks have been previously disclosed. Typically, a MOF includes metal clusters linked together in a periodic fashion by linking ligands that increase the distance between the clusters to give a net-like structure. MOFs based on the same net topology (i.e. underlying symmetry and connectivity) are described as “isoreticular”. Li et al. disclosed a metal-organic framework (referred to a MOF-5) formed by diffusing triethylamine into a solution of zinc(II) nitrate and benzene-1,4-dicarboxylic acid (H.2BDC) in N,N-dimethyl-formamide/chlorobenzene followed by deprotonation of H2BDC and reaction with the Zn2+ ions (Li, Hailian, Mohamed Eddaoudi, M. O'Keeffe and O. M. Yaghi, “Design and synthesis of an exceptionally stable and highly porous metal-organic framework,” Nature, Vol. 402, pp. 276-279 (Nov. 18 1999)). The MOF-5 framework was found to comprise an extended, porous network having a three-dimensional intersecting channel system with 12.9 Å spacing between centers of adjacent clusters. Although the MOF-5 crystalline structure possesses a number of desirable characteristics, the MOF-5 framework is formed in relatively low yield. Moreover, the MOF-5 structure appears to be limited to a single benzene ring as a linkage between adjacent Zn4(O)O12C6 clusters. U.S. Pat. Appl. 20030004364 (the '364 application) expands and enhances the work disclosed in Li et al by providing the preparation for a number of isorecticular metal-organic frameworks. The '364 application recognizes an improvement by requiring that the linking ligand include a phenyl with an attached functional group. It should also be appreciated that the linking ligands in both Li et al and the '364 application are polydentate charged ligands. Although the '364 application provides insight into the tailoring of metal organic frameworks, further improvement is still needed for identifying those molecular components which most effectively increase surface area.
Researchers have also attempted to formulate frameworks having longer links between adjacent clusters by using polytopic N-donor ligands. Synthesis of open frameworks by assembly of metal ions with di-, tri- and polytopic N-donor organic linkers such as 4,4′-bipyridine has produced many cationic framework structures. Although such synthesis may produce frameworks with varying pore sizes, attempts to evacuate/exchange guests within the pores often result in the collapse of the host framework making the practical utility of such frameworks limited.
Accordingly, there is a need in the prior art for porous structures with increased adsorption and in particular, for methods of making such structures in a systematic manner.